Stable Lanthanide Metal–Organic Frameworks with Ratiometric Fluorescence Sensing for Amino Acids and Tunable Proton Conduction and Magnetic Properties
Jun‐Jie Hu, Yuguang Li, He‐Rui Wen, Sui‐Jun Liu, Yan Peng, Cai‐Ming Liu
Abstract
Four new isostructural lanthanide metal–organic frameworks (MOFs), namely {[Ln(DMTP-DC)1.5(H2O)3]·DMF}n [H2DMTP-DC = 2′,5′-dimethoxytriphenyl-4,4″-dicarboxylic acid; LnIII = EuIII (1), GdIII (2), TbIII (3), and DyIII (4)], have been synthesized and characterized. Single-crystal structure analysis reveals that 1–4 are three-dimensional Ln-MOFs with rich H-bonding of coordinated H2O molecules in the network channels. The X-ray diffraction patterns indicate that Ln-MOF 1 displays good stabilities in organic solvents and aqueous solutions with distinct pH values. Both 1 and 3 show characteristic emission of LnIII ions. Ln-MOF 1 can be used as a ratiometric fluorescence sensor for arginine and lysine in aqueous solution, and the detection limits are 24.38 μM for arginine and 9.31 μM for lysine. All 1–4 show proton conductivity related to relative humidity (RH) and temperature, and the maximum conductivity values of 1–4 at 55 °C and 100% RH are 9.94 × 10–5, 1.62 × 10–4, 1.71 × 10–4, and 2.67 × 10–4 S·cm–1, respectively. The value of σ increases with the decrease in ionic radius, indicating that the radius of the LnIII ions can regulate the proton conductivity of these MOFs. Additionally, 2 exhibits a significant magnetocaloric effect (MCE) with a magnetic entropy change (−ΔSm) of 18.86 J kg–1 K–1 for ΔH = 7 T at 2 K, and 4 shows weak field-induced slow relaxation of magnetization. The coexistence of good fluorescence sensing capability, attractive proton conductivity, and relatively large MCE in Ln-MOFs is rare, and thus, 1–4 are potentially multifunctional MOF materials.